Introduction: DIY : Lego UV LED Flashlight / Homemade Pet Urine Detector

This is an easy ( No Soldering Required ), fun, and cheap way to make a great UV LED Flashlight out of Legos. This also doubles as a homemade Pet Urine Detector ( compare prices ). If you have ever dreamed of making your own homemade Lego Flashlight, then here you go! This is also a great DIY craft project for Kids as well.


Battery Holder AA or AAA with wires already attached ( easy find on Amazon ). These Almost Always have an Output Voltage of 3V DC. This is what we want, as to simplify the circuit without having to add a resistor.

Batteries... Same size as holder... :/

On / off Push button OR slide switch for electronics. Easier if it has wires already attached. If not cool...

22 AWG crimp "butt" connectors. Don't be cheap get the heat shrink :) JK... the others work fine also.

5mm UV LED ( Fwd. Current of about 2.4 - 3.2V DC ) Don't be daunted by the technical info, pretty basic stuff for LED's.

22 AWG Wire.

Super Glue, or Hot Glue... With gun of course.

Cool Lego Pieces :) Translucent are the best.

Step 1: Putting Together Your Circuit

We will be using the butt connectors for the permanent connections instead of soldering. This is much easier and safer for Kids doing this project. These are as easy as sliding the stripped wire into the tube and crimping down. If you need a more detailed explanation, there are numerous sites that can show you.

These butt connectors will connect from Battery Positive on the battery holder to the LED, to the switch, back to battery negative.

Make sure all of your connections are secure so they do not come loose.

The one tricky part of using butt connectors for this project are the small connection pins on the slide switch. This just takes a little more finesse. I will explain that in a later step.

Step 2: Lay It All Out

The picture here shows you how to lay out your circuit. These components can actually be put in any order, so if you are familiar with these circuits, move them around to your liking.

Like I said before, keep polarity correct at the positive side at the LED ( longer Lead ) from the positive wire of the battery holder.

Crimp the LED to the post. Wire.

Crimp the negative lead of the LED to one of the END and middle pins of the switch. You do not need the other end of the switch, just TWO pins for a regular on and off function as it is a SPDT switch ( look it up) for now JUST use the middle pins, and ONE End pin...
As in the picture, strip back some of the sleeve of the butt connector so you can slide it onto the switch pin. If you don't, it will probably not fit.

After you have the connectors on the switch pins ( same if you use a push button switch [only two pins... on / off ] ), Connect the other end of the switch to the battery negative wire.

You have completed your circuit. :)

It's up to you to decide what Lego Pieces to put your LED circuit in.

Step 3: Build Your Legos

I know we ALL have extra Legos laying around. You can build this any way you like. All you have to do is super glue or hot glue your circuit into your Lego build ( be creative )... Be sure to position it a couple times before you commit to gluing it. Make sure the switch is in an accessible place. Aaaaaand... Make sure that your connections are not inundated with your glue, or it might open the circuit, not allowing the electricity to flow, not lighting your LED. When gluing I suggest gluing the CASE of the switch, and battery holder to your Legos and leave any metal of the wire or connectors or LED alone. They will NOT shock you, they just should not get dirty or glue on them. Aaaanyway...

Let's see... Follow polarity, make sure your butt connectors are secure, don't glue wires or the LED leads
( you CAN glue the plastic part of the LED to your Lego Pieces. The heat from the LED at a low voltage will NOT melt the glue ). And be creative. These pics are just some ideas.

Have fun and enjoy